Cast is a Test + Conversion so to speak - so any object will do.
If you plug in an object reference that is of Point Light Blueprint type, it will succeed. If you plug in something else, it will Fail.
If the above makes little sense to you, perhaps explain what you’re trying to achieve. Or what you expect to happen here.
In most cases the editor expects you to specify which instance of the Light you mean - perhaps there’s more than one…
It’s a common misconception that Casting is a method of communication and allows one to find an object in the world. It’s your job to tell the editor which object you mean so you can cast.
Think of casting as of a test. Is this actor (wildcard) of the type I want? If it is, start treating it as that actor type.